1. Field of the Invention
This invention relates to a process for producing a precision, cylindrical cartridge filter, by subjecting a web composed of conjugate melt-blown microfine fibers to forming under heat into a shape of a cartridge filter.
2. Field of the Related Art
A cartridge filter using melt-blown fibers (hereinafter often referred to as cartridge) is disclosed in Japanese patent application laid-open No. Sho 60-216818. Further, Japanese patent publication No. Sho 56-43139 discloses a process for producing a cylindrical fiber aggregate by after forming a web of conjugate short fibers by means of a carding machine, rolling up the web under heat. Still further, Japanese patent publication No. Sho 56-49605 discloses a process of inserting a sheet-form material having small pores into an intermediate portion of a cartridge.
However, the melt-blown fibers of the above Japanese patent application laid-open No. Sho 60-216818 are composed of a single component, a material having almost no bond of fibers to one another and retained by mechanical snarl or entanglement to one another and further having weak strength in the form of a cartridge filter; thus the fibers employ a structure provided with a central supporting member, so that filter production is not simple. Further, the fibers have a percentage of void as high as 80 to 90% so that it is difficult to retain the outer diameter of the cartridge filter even when it is used under low pressure. In particular, the smaller the particles to be filtered, the smaller the fiber diameter of the filter, so that notable shrinkage of the outer diameter of the filter occurs, resulting in anxious filtrability.
In the case of the cartridge using conjugate fibers, disclosed in the above Japanese patent publication No. Sho 56-43139, since short fibers cut to a definite length are processed, the fineness (denier) and the cut length of the fibers used have been naturally restricted. On the present level, stabilized carding of fibers of 1 d/f (denier/filament) or less is so difficult that it has been impossible to produce a cartridge capable of filtering off impurities of 10 .mu.m or less. In order to overcome such a drawback, the above Japanese patent publication No. Sho 56-49605 discloses a process of inserting a sheet-form material having small pores into an intermediate layer of a cartridge. However, the difference between the diameter of the fibers composing the sheet-form material and the diameter of the fiber of the cartridge body is so large that it is presumed that the layer of the sheet-form material works as a rate-determining step in a mechanism of surface layer filtration as in the case of membrane pleat filter. Further, such a sheet-form material is of a different kind of a stock from that of the stock constituting the cartridge, so that there have been problems of ply separation, etc. in the case of filtration of a high pressure and high viscosity fluid. Further, in the case of using short fibers, in order to improve the handling of the processing, an antistatic agent (oiling agent) has so often been attached to the fibers. Thus, such a cartridge prepared from the fibers having the oiling agent, the oiling agent dissolves in a filtrate and bubbles at the initial stage of filtration, so that it is the present status that the cartridge can be expressly washed and used in the field of foods or in a precision filtration.